1. Field of the Invention
The present invention relates to composite anode active materials, methods of preparing the same, and anodes and lithium batteries containing the anode active materials. More particularly, the invention is directed to composite anode active materials including an intermetallic compound.
2. Description of the Related Art
In an effort to achieve high voltages and energy densities, research and development has been extensively conducted into non-aqueous electrolyte secondary batteries using lithium compounds as anodes. Specifically, metallic lithium has become the subject of intense research due to its ability to impart high initial battery capacity. However, when metallic lithium is used as an anode material, large amount of lithium can deposit on the surface of the anode in the form of dendrites, which may degrade the battery charge and discharge efficiency or cause internal-shorts between the anode and the cathode. Furthermore, lithium is sensitive to heat and impact, and is prone to explosion due to its instability and high reactivity. These problems have tended to limit the commercialization of batteries with metallic lithium. In order to eliminate these problems with the use of metallic lithium, carbonaceous materials have been proposed for use as anode materials. Carbonaceous anodes aid in redox reactions such that lithium ions in an electrolytic solution intercalate/deintercalate in the crystal lattice structure of the carbonaceous material during the charge and discharge cycles. These anodes are referred to as a “rocking chair” type of anodes.
The carbonaceous anode has contributed to the use of lithium batteries by overcoming various disadvantages associated with metallic lithium. However, electronic equipment is becoming smaller and lighter in weight, and the use of portable electronic instruments is becoming more widespread, making the further development of lithium secondary batteries having higher capacities of interest.
Lithium batteries using carbonaceous anodes have low battery capacities because of the porosity of the carbonaceous anodes. For example, graphite, which is a highly crystalline material, when made into a structure in a form of LiC6 by reacting with lithium ions, has a theoretical specific capacity of about 372 mAh/g. This is only about 10% that of metallic lithium, which has a capacity of about 3860 mAh/g. Thus, in spite of many problems with conventional metallic anodes, studies for improving battery capacity using metallic lithium as an anode material are being carried out.
Generally metal and/or metalloid containing materials such as silicon or tin, or a lithium-containing alloys such as lithium-aluminum, lithium-lead, lithium-tin, or lithium-silicon alloys have higher electrical capacities than carbonaceous materials. However, when metals or alloys of two or more metals are used, formation of lithium dendrites is likely to occur. In addition, considerable volume change due to expansion and contraction of the metals can also occur, resulting in poor coulombic efficiency and a reduction in battery cycle life.
One proposed solution to avoid these problems is to use silicon in combination with graphite or other carbonaceous materials. While the lifespan of these batteries increases to a certain extent, their initial charge and discharge efficiencies remain poor. This is because of poor graphitization of the carbonaceous materials. The degree of graphitization is reduced when the edge portions of crystalline surface of the graphite are substantially exposed during the course of mixing silicon with carbonaceous materials. Poor graphitization of carbonaceous materials can result in faster decomposition of the electrolyte during battery charging.
In addition, the use of carbon-based anodes can present further problems. For example, it is difficult to obtain a high degree of graphitization while attaining a silicon composite of carbonaceous material. One proposed solution is to use metallic components instead of a carbonaceous materials in combination with silicon to increase the initial coulombic efficiency. While the use of metallic components may enhance the initial coulombic efficiency, the metals tend to form intermediate phases with lithium, resulting in the same problem of repeated volume expansion and contraction when lithium is introduced and withdrawn from metallic components during battery charge and discharge.
Another proposed solution is to use metallic materials that do not form lithium alloys. However, analysis of binary alloy phase diagrams shows that metallic materials that do not tend to form alloys with lithium form alloys with silicon.
For example, as shown in the phase diagrams of FIGS. 6 and 7, a metallic material such as nickel does not form intermediate phases with lithium. However, it alloys with silicon to form various intermediate phases.
The formation of intermediate phases requiring a consumption of silicon and metallic materials such as nickel, accompanied during the course of mechanical milling or thermal treatment for producing silicon composite, causes a reduction of silicon contents within the composite. Thus, it results in a reduction of battery capacity.